The invention relates to a process and an electric circuit for digitally indicating electric measuring and/or test values.
It frequently turns out to be advantageous in many areas of engineering to indicate measuring and/or test values digitally. In recent times for indicating measuring and/or test values on test stands for tires, motors, vehicles, etc., the trend has been toward digital indication. Satisfactory commercial indicators, however, are still not available.
The difficulty with existing indicators is that periodic and stochastic small fluctuations are superimposed on the measured test values which are obtained. For analog devices such as those which have indicator needles which move in front of a scale, such fluctuations are largely averaged out through the inertia of the mass of the measuring mechanism whenever they are above the border frequency of the measuring mechanism. Furthermore, the impression for the observer is improved through the fact that indicator instruments have only a limited resolution capability, which is predetermined by the shaping of the pointers and the width of the marks of the scale. Slight fluctuations of the measuring values are thus reduced by the characteristics of the measuring mechanisms to an extent which imparts to the observer a "quiescent" appearing indication.
In the case of use of digital indicating devices, generally the demand is made that the resolution is to be at least equally as good, in most cases better than in the case of pointer instruments. A three digit indication of 000 to 999 cannot be avoided as a rule. Consequently fluctuations of the measuring value of only 0.1%0 will influence the last digit and fluctuations of measuring values of 1% will influence the last two digits. For the observer of the indicating arrangement this is unsettling since he finds it difficult to average out these digital changes, optically. In some cases this will even be impossible, since arithmetical operations would have to be carried out for this purpose. Moreover, the internal rate of the measuring cycle of the digital indicating instrument, e.g., a digital volt meter, is superposed on these fluctuations of the measuring values, that is to say the digital indicating instrument measures at points in time which are independent of the course of the fluctuations. As a result there develops a beat in the indication which makes it impossible for the observer to average at all.
In order to achieve a quite "flickerless" digital indication, it is necessary to average out the fluctuations of the measuring signals electronically. For this purpose, circuits are already obtainable commercially which smooth out the higher frequency portions by using low-pass filters. Whenever one also wishes to filter out fluctuations at lower frequencies, the threshold frequency of the low-pass filter must be reduced still further, which has a disturbing effect with regard to the build-up times, and adjusting times. The adjusting time of the low-pass filter arrangement increases with decreasing threshold frequency.
The consequence of this increase is that a relatively long time passes after lock-on of a measuring and/or test value on the digital indicating instrument, especially on digital volt meters, or else in the case of fluctuations of the measuring and/or test value in case of a change of testing conditions, until the digital indication changes to the new value. The digital volt meter behaves then just like an excessively strongly damped indicator measuring mechanism, i.e., the adjusting time is for most purposes unreasonably long. The disadvantage of periodic fluctuations of the indications have been largely avoided, but the price is a long adjusting time.
Therefore, it is an object of this invention to create a process and an electric circuit for the digital indication of measuring and/or test values which provides on the one hand a quasi-constant digital indication and on the other hand a short build-up time.
According to this invention, this object is achieved in that the electric signals corresponding to the measuring and/or test values are conducted simultaneously through the two channels, one of which has a higher adjusting time than the other, and are compared to one another simultaneously. In the case of a differential signal occurring because of a fluctuation of the measuring and/or test value, the signal appearing at the exit of the channel with the higher adjusting time and used for the indication is adjusted to the new measuring and/or test value quickly as a function of the differential signal.
In addition, it is advantageous if the rate of measurement of the indicating arrangement is increased in dependence on the differential signal which is obtained from the two outlets of the channels with variable adjusting time when strong fluctuations or jumps of the measuring and/or test values occur.
This is accomplished by an electric circuit for the digital indication of measuring and/or test values with a digital indication arrangement, especially a digital volt meter and a preceding low-pass filter whereby two low-pass filters with variable adjusting times are connected in parallel to the inlet for the measuring and/or test values. The outlets of the low-pass filters are coupled at a comparator circuit, the output of which is connected by way of a threshold value step circuit with the output of the low-pass filter with the higher adjusting time to which the inlet of the digital indicating arrangement is coupled.
The threshold value step circuit in this case can be adjusted in such a way, that in the case of slight fluctuations, the electric signal corresponding to the measuring and/or test values is connected only with the digital indicating arrangement by way of the low-pass filter with the higher adjusting time.
For this purpose the outlet of the comparator circuit can be connected with a capacitor at the output of the low-pass filter with the higher adjusting time, which delivers its charge to the digital indicating arrangement by way of the impedance transformer.
The comparator can be developed as a differential amplifier, the first input of which is connected with the output of the low-pass filter with the lower adjusting time, the second input of which is connected via the impedance transformer with the output of the low-pass filter with the higher adjusting time and the output of which is connected by way of Zener diodes to the capacitor at the inlet of the impedance transformer.
In order to raise the count rate of the digital indicator in response to particularly strong fluctuations or jumps of the electric measuring and/or test values, an electric circuit can be provided, by way of which the output of the comparator circuit drives the digital indicating arrangement, especially by way of a threshold value step. This driving circuit for the digital indicating arrangement can include a voltage controlled oscillator, the output signal of which constitutes the timing pulses for the digital indicator. The threshold value in this driving circuit can be made up of two Zener diodes connected in parallel, one of which is connected by way of an inverter with a common connecting point which is located at the input of the oscillator. Thus, in the case of a change in the signal because of a strong measuring and/or test value fluctuation or of a jump of this value, a short adjusting time is achieved and after the signal value has been brought to the new value, the adjusting time will be correspondingly higher so that an average which creates a smooth indication is produced. In other words, in the case of a strong change of the measuring and/or test value, the low-pass filter of the low order, respectively of high threshold frequency becomes effective and quickly pulls the signal value, which is used for the indication, to the new value and as soon as this new value is achieved, the low-pass filter with the high order, respectively a low threshold frequency, becomes active, by way of which then only the signals corresponding to the measuring respectively test values are fed on to the digital indicating arrangement.
As a result of the additional driving circuit, the measuring rate (indicating rate per unit of time) of the digital volt meter, respectively of the indicating arrangement can be controlled simultaneously in such a way, that with an approximately constant average value of the measuring value, there is a relatively long period of time between the two measurements and in the case of a signal jump, the measuring rate of the digital indicating arrangement is increased considerably so that the indication adjusts itself quickly to the new value and moreover the observer can recognize immediately, as a result of the raised sequence of indications on the indicating arrangement, that a jump in measuring value has occurred. Whenever the indication has steadied to the new value, the measuring rate, the indicating rate again becomes longer. Thus, an acceleration of the measuring rates or of the indicating rates is shown to the observer of the indicating arrangement, just as in the case of an indicator instrument in correspondence with the acceleration of the pointers in the case of jumps of the measuring value. This is a considerably improved aid for the observer as compared to the constant indication, respectively measuring rate, which merely shows the variable differences between the indicating values of successive measurements. The invention guarantees the avoiding of any shortcomings occurring in the case of the known digital indicating devices, which shortcomings have been described in the beginning and conveys to the observer an improved indication of the measurement.